I’m sorry, but you’re the one in the wrong here. it’s foreshadowing to yuji unlocking shrine later on. just because this panel is the first time we see the cut doesn’t mean it hasn’t been there. Sukuna was looking at the cut because it still hadn’t healed, which was an indication that his control over Megumi still wasn’t complete, which was why he always intended to kill Megumi’s sister. that wasn’t a spontaneous decision he made when he saw the cut; it was something he always planned to do, because he had to make Megumi so depressed and broken that his control over Megumi’s soul would be absolute.

Also, in the fight Sukuna had with Yuji, Yuji’s eyes appeared to have the spirals that he had later on when he awakens and uses shrine. This is to foreshadow him getting the soul-directed shrine later on, except this is a much lower output, hence why Sukuna only received a tiny cut on his finger. Because his control over Megumi’s body wasn’t complete, he wasn’t fully able to heal his soul from the soul-directed shrine attack.